Polybutadiene is generally superior to other rubbers in its fuel efficiency but is inferior in its processability. However, since fuel efficiency and processability are in a conflicting relation and when one of them is intended to be improved the other performance deteriorates, various improvements have been made.
For example, there has been reported a polybutadiene composition for tire in which both fuel efficiency and processability are improved by specifying a ratio (Tcp/ML) of 5% toluene solution viscosity (Tcp) to a Mooney viscosity (ML) of a polybutadiene synthesized by using a cobalt catalyst (Patent Document 1).
Further, an attempt of further improving both fuel efficiency and processability by specifying, in addition to a ratio (Tcp/ML) of 5% toluene solution viscosity (Tcp) to a Mooney viscosity (ML) of a polybutadiene synthesized by using a cobalt catalyst, a velocity dependence index (n-value) of the Mooney viscosity has been made (Patent Documents 2 and 3).
Rubber compositions comprising carbon black in addition to a rubber component that is obtained by blending a natural rubber which exhibits excellent tensile strength and tear strength, a butadiene rubber which improves flex crack growth resistance and the like have been conventionally used for sidewalls of tires, but in recent years, an attempt of reducing the amount of rubbers in sidewalls has been promoted in order to lighten tires and further improvements of durability of tires such as tensile strength, tensile elongation, tear strength and flex crack growth resistance of sidewalls are required.
In rubber compositions used for a clinch of a tire, a butadiene rubber has been blended for improving flex crack resistance in addition to a natural rubber exhibiting excellent tear strength, and further carbon black has been used for improving weather resistance and a reinforcing property. Also, fuel consumption has been reduced by reducing rolling resistance of tires. The further reduction in fuel consumption has been more and more strongly required, so that not only a rubber composition for tread but also a rubber composition for clinch are required to have excellent fuel efficiency.
As a method for satisfying fuel efficiency of a rubber composition, a method of reducing a content of fillers such as carbon black, silica and the like is known. However, in this case, there is a problem that the strength of rubber composition decreases and abrasion resistance deteriorates.
Patent Document 4 discloses a rubber composition which can improve fuel efficiency by compounding silica with different particle diameters. However, with respect to improvement of low heat build-up property, there is still room for improvement.